Because many cellular processes are adversely affected by acidic pH, mammalian cells must regulate intracellular pH above electrochemical equilibrium. To achieve this, steady state intracellular pH is set by a careful balance of transmembrane fluxes of proton equivalents through specialized transport systems, nonspecific leaks and metabolic acid production. In nucleated cells, these transport systems include sodium/hydrogen exchange and sodium-dependent and independent chloride/bicarbonate exchange systems. Regulation of intracellular pH will be studied in the human promyelocytic leukemic cell line HL60. The HL60 cell uses the transport systems mentioned above to regulate intracellular pH, is amenable to biophysical, biochemical and molecular biological studies of transport kinetics and displays physiologically relevant developmental changes in pH regulation and exchange kinetics. The major goal of this project is to study the kinetics of these exchanges to establish whether they follow ping- pong or simultaneous kinetics and to develop predictive models of each transport system. Once these models are established and tested, they will be used as a framework to study the regulation and developmental changes of the exchange systems. This work will also provide a basis for future studies of the structure-function relationship of the exchange molecules. A combination of optical measurements of intracellular ion activities (using in situ generated fluorescent probes) and radioactive tracer techniques will be used together with theoretical tools developed in the study of red blood cell anion exchange. The use of these techniques will allow a comprehensive study of intracellular pH regulation in a model system where individual exchanges are well characterized.